In applications such as Digital Radiography (DR), Flat Panel Detectors (FPDs) can be used to indirectly acquire X-Ray images. FPDs typically comprise a matrix of individual pixel sensor circuits and use indirect conversion from X-ray photons to optical photons using a “phosphor” (not shown) over the entire area of the detector. Typical scintillation materials used in X-ray imaging are phosphors such as structured Cesium Iodide (CsI(TI)) and Gadolinium OxySulfide (Gd2O2S(Tb)), also known as GadOx or GOS. Optical light photons from the phosphor that reach photodiodes within respective pixel circuits are converted into single electron-hole pairs, and the resulting charge is stored within the pixel circuits, using either discrete capacitors and/or parasitic capacitance.
FIG. 1 shows a circuit diagram for a 3×3 pixels portion of a conventional amorphous silicon backplane silicon FPD which uses an Active Matrix (AM) sequential addressing scheme. FIG. 2 shows a cross-section of a first example of an individual pixel for the matrix portion shown in FIG. 1. FIG. 3 shows a cross-section of a second example of an individual pixel formed using a Silicon-on-Glass process. As shown in FIGS. 2 and 3, the individual pixel comprises a single detecting diode electrically connected to a corresponding thin-film switching transistor.
In operation, charge that is accumulated (in capacitors not shown) during an X-ray exposure of a pixel circuit is readout by sequentially addressing a row gate electrode, which turns the switching thin-film transistor of the pixel circuit from an off-state into an on-state. The resulting charge, which is proportional to the X-ray intensity at that pixel position, is then conducted to a respective readout circuit for a column.
U.S. Pat. No. 7,323,692, Rowlands and Zhao describes the use of an avalanche layer in X-Ray imaging indirect conversion detectors, using an active matrix addressing scheme. Here X-rays enter a phosphor such as Caesium Iodide (CsI), generate a shower of optical photons, which further generate electron-hole pairs via photoelectric effect in the avalanche layer, which may be amorphous Selenium or amorphous Silicon.
Nonetheless, these detectors are limited by Detection Quantum Efficiency (DQE) at low dose of incident electromagnetic radiations.
Thus, high-quality large-area X-ray images of high-risk category subjects, such as the pregnant, children, or the obese, require high dose levels to achieve a useful image. If the exposure risk is considered too high to proceed, an alternative, inferior, diagnostic method must be used.
Separately, in virtual reality applications, gesture recognition and eyeball tracking is currently achieved by imaging the body or the eyes under illumination, typically in the infra-red wavelength range. Nonetheless, near infra-red imaging sensors, especially for eyeball tracking, are limited by scene illumination intensity and speed limited by acquisition/integration time.
It is an object of the present invention to mitigate these shortcomings.